A number of months ago I purchased a Photolume QS decoder on eBay. I hooked it up and was underwhelmed by the sound, it wasn't doing that much and had a faint background hum. Latter when I had the time I replaced the 1000 uF power supply filter capacitor with a new 3300 unit. The regulated supply was putting out less than the required 25V, so I had to change a resistor value in the supply to obtain exactly 25V. The lower value electrolytic coupling capacitors were replaced with film types at the same time. New Input and Output jacks were added. The old Phenolic RCA jack strip has the jacks spaced much too close together, most of the cables I use wouldn't fit in them properly. The box then sat for several more months.
Recently I hooked up the box again and have been blown away by the sound. I use mainly the surround mode for stereo enhancement. The sound is stunning, stereo spread around the room in a horseshoe pattern as expected but even better than expected. At times it sounds likes discrete quad, with all four channels active. Mono sources sound good too, all upfront and imaged right between the front speakers with nothing audible from the rear. Lacklustre stereo sources now sound like decent stereo, imaging mainly from the front.
I also own a Sansui QSD-1, and although I love the quality of the build, I've always maintained that it colours the sound considerably. I believe that the band-pass filter networks and the output phase shift circuit are the cause of this coloration. The Photolume is free of both of these and as such it has a much cleaner signal path.
The only downside is the unit looks homemade, it was sold as a kit after-all. The complete construction article was published in the December 1976 issue of Popular Electronics. I would of purchased the kit back then but was a poor student at the time.
In future I may follow some of R. Scott Varner's advice in the (Vol.6, No. 2) Autumn 1984 issue of the MCS Review "A VARIO-MATRIX SURVIVAL GUIDE". Add a pot for to make the surround and hall function variable. Replace the fixed resistors of the photolume with separation trim pots as used in the QSD-2, although the unit functions so well with the resistors chosen by Photolume it's hard to imagine being able to tweak it further. And also ATTACH]27747[/ATTACH]
replace the rest of the electrolytic coupling capacitors.